Hydrogen storage alloy

ABSTRACT

A hydrogen storage alloy having a body-centered cubic structure phase capable of storing and releasing hydrogen as its main phase, and a composition of the general composition formula: Ti (100-a-0 4b) Cr (a-0.6b) V (b-c) M c , wherein 20≦a (at %)≦80, 0≦b (at %)≦10, and 0≦c (at %)&lt;5; and M is at least one element of Mo and W.

TECHNICAL FIELD

[0001] The present invention relates to a hydrogen storage alloy capableof repeatedly carrying out the absorption and release of hydrogen.Particularly, the present invention relates to a BCC-based hydrogenstorage alloy having theoretically a high capacity for hydrogen storage.Further, the present invention especially relates to a hydrogen storagealloy having highly practicable properties, including, for example, notonly quantitatively excellent hydrogen adsorption and desorptioncharacteristics within practical pressure ranges and temperature rangesbut also a capacity of adsorbing and desorbing hydrogen in quite greatamounts per unit weight, together with a relatively inexpensiveproductivity, etc.

RELATED ART OF THE INVENTION

[0002] At present, there have been fears of not only acid rain due toincreasing NO_(x) (nitrogen oxides) but also the global warming due tosimilarly increasing CO₂ in association with an increase in consumptionof fossil fuel such as petroleum. Such environmental destruction hasbecome a serious problem. Therefore, our attention has been greatlyconcentrated on development and practical application of various kindsof clean energy which is friendly to the earth. As a part of this newenergy development, practical use of hydrogen energy is given. Hydrogen,which is a constituent element of water inexhaustibly present on theearth, is not only producible by using various kinds of primary energy,but also utilizable as a fluid energy in place of conventionally usedpetroleum without the risk of destroying the environment because itscombustion product is only water. In addition, unlike electric power, ithas excellent characteristics such as its relatively easy storage.

[0003] In recent years, therefore, investigation has been activelyconducted involving hydrogen storage alloys as storage and transportmedia for hydrogen, and their practical application has been expected.Such hydrogen storage alloys are metals/alloys that can absorb oradsorb, and release hydrogen under an appropriate condition and, by theuse of such alloys, it is possible to store hydrogen not only at a lowerpressure but also in a higher density as compared to the case of theconventional hydrogen cylinders. In addition, the hydrogen volumedensity thereof is nearly equal to or rather more than that of liquid orsolid hydrogen.

[0004] Among these hydrogen storage alloys, AB₅ alloys such as LaNi₅ andAB₂ alloys such as TiMn₂ have been put into practical use until now, buttheir hydrogen absorbing capacity is still insufficient. Therefore, asproposed, for example in Japanese Unexamined Patent Publication (Kokai)No. 10-110225 (JP, A, 10-110225 (1998)), metals having a body-centeredcubic structure (hereinafter referred to as “BCC” or “BCC type”) (e.g.,V, Nb and Ta), and BCC type alloys thereof (e.g., TiCrV-based alloys,etc.) have been mainly examined in recent years because the number ofhydrogen absorbing sites is great and the hydrogen absorbing capacityper unit weight of the alloy is an extremely large value as large asH/M=ca. 2 wherein H is an occluded hydrogen atom and M is a constituentelement for the alloy (about 4.0 wt % in case of V with an atomic weightof around 50, etc.).

[0005] With regard to alloys wherein Ti and Cr are comprised, assuggested in JP, A, 10-110225, when the admixture ratio of theconstituent metals in alloys comprised of only Ti and Cr is brought tosuch an extent that it will be conductible to absorb and releasehydrogen at a practicable temperature and pressure (i.e., the atomicratio of Ti is set at 5<Ti (at %)<60), a temperature range for forming aBCC structure becomes very narrow between a melting point of the alloyand a temperature at which a C14 crystal structure is formed as alsoapparent from FIG. 2 (phase diagram for the Ti—Cr binary alloy).Consequently, other C14 crystal structure phases which are differentfrom BCC are formed at 90 wt % or more in the alloy and it is verydifficult to produce the BCC. Therefore, the aforementioned TiCrV-basedalloys are products obtained by admixing V as an element highly capableof forming BCC together with both Ti and Cr so as to attain the BCCstructure in a more stable fashion and at a lower temperature. It hasbeen reported that it is difficult to form the BCC as their main phaseeven by application of heat-treatment unless the amount of V is at least10% or more and as a result no good hydrogen adsorption and desorptioncharacteristics are obtainable.

[0006] Further, a Ti—Cr-based alloy (comprised of 5 or more elements)having the formula: Ti_((100-x-y-z))Cr_(x)A_(y)B_(z), wherein A is onemember selected from V, Nb, Mo, Ta and W, and B is two or more membersselected from Zr, Mn, Fe, Co, Ni and Cu, and its crystalline structureis BCC, is disclosed in Japanese Unexamined Patent Publication (Kokai)No. 7-252560 (JP, A, 7-252560 (1995)), wherein it is pointed out thatthe aforementioned admixture of 5 or more elements is essential foracquiring the aforementioned BCC.

[0007] However, there are still problems: since V to be admixed with theaforementioned alloy has an atomic weight approximately similar to thatof Ti or Cr, it may be admixed at an elevated quantity without reducingits hydrogen storage capacity per unit weight of the alloy product somuch, but because it is very expensive, especially highly pure one(99.99% purity) employed for such an alloy is extremely expensive, theprice of the alloy product results in a very high level, whereby alloycosts will increase for absorbing and storing an equal amount ofhydrogen.

[0008] Therefore, for inexpensive alloys free of using precious V,Mo—Ti—Cr-based and W—Ti—Cr-based alloys are proposed wherein Mo or W isadmixed as, like V, an element highly capable of forming BCC with bothTi and Cr. However, for these Mo and W, as suggested in JapaneseUnexamined Patent Publication (Kokai) No. 10-121180 (JP, A, 10-121180(1998)), it has been reported as follows: such alloys are not made intoBCC forms even by application of heat-treatments when Mo and/or W isadmixed at 0 at %, nor is BCC obtainable as the main phase when Moand/or W is admixed at a low level, similarly to the above V.Accordingly, no good hydrogen absorption and desorption characteristicswill appear. There are also problems: when the amounts of Mo and W to beadmixed increase, the hydrogen absorbing capacity per unit weight ofsuch alloys will be reduced because of their large atomic weight, and incase where these hydrogen storage metal alloys are used as energysources for automobiles, bicycles, etc. in the form of hydrogen gasstorage tanks and nickel hydrogen batteries, including fuel batteries,their weights would unavoidably increase when an attempt is made atattaining a necessary electric power and hydrogen-supplying performance.

[0009] In view of the foregoing points, the present inventors have paidmuch attention to the aforementioned problems and, as a result,succeeded in the present invention. An object of the present inventionis to provide a hydrogen storage metal alloy which is (i) producible inthe aforementioned form having BCC main phases even if the level ofprecious V, or Mo and W which each lead to a decrease in hydrogenabsorbing capacity per unit weight, is made null or as minimal aspossible, also (ii) excellent in view of its cost and hydrogen absorbingcapacity per unit weight and (iii) highly practicable.

SUMMARY OF THE INVENTION

[0010] In order to solve the aforementioned problems, the presentinvention provides a novel hydrogen storage alloy for adsorption,storage and desorption of hydrogen. According to the present invention,the novel hydrogen storage alloy has the following characteristics:

[0011] (1) it has as its main phase a body-centered cubic structure-typephase capable of absorbing, storing and releasing hydrogen, and

[0012] (2) it has a composition of the following general compositionformula:

Ti_((100-a-0 4b))Cr_((a-0.6b))V_((b-c))M_(c)

[0013] wherein 20≦a (at %)≦80, 0≦b (at %)≦10, and 0≦c (at %)≦5; and M isat least one element of molybdenum (Mo) and tungsten (W).

[0014] Such characteristics lead to the following:

[0015] An amount of expensive V contained therein is partially replacedwith at least one element selected from the group consisting of Mo and Wpotently capable of forming a BCC structure together with Ti and Cr inthe same manner as V, whereby a decrease in hydrogen storage capacityper unit weight, brought about by the inclusion of Mo or W, can berestricted to a relatively minor one at a relatively low cost.

[0016] As a result, advantageously practicable hydrogen storage metalalloys well-balanced between the cost and the hydrogen storage capacityper unit weight can be produced, provided that other elements can beoptionally admixed as long as their admixture does not affect greatlythe aforementioned properties of the hydrogen storage metal alloys.

[0017] It is preferred that the hydrogen storage alloys of the presentinvention are those wherein an element, X, having an atomic radiuslarger than that of Cr but smaller than that of Ti may be contained atan atom % concentration, d (at %), ranging within 0≦d (at %)≦20.

[0018] As a result thereof, the element X can be admixed the atomicradius of which is larger than that of Cr but smaller than that of Ti,thereby inhibiting the formation of a C14 (Laves phase) structure so asto extend a temperature range for forming a BCC structure phase in placeof the aforementioned C14 (Laves phase) structure, with the result thatthe hydrogen storage metal alloys can be produced with the BCC structurephase in a stable fashion even at low levels of V, Mo and W, which eachhave a potent BCC structure-forming capability with both Ti and Cr.

[0019] It is preferred that the hydrogen storage alloys of the presentinvention contain at least one or more elements (T) selected from Nb,Ta, Mn, Fe, Al, B, C, Co, Cu, Ga, Ge, Ln (a variety of lanthanoidmetals), N, Ni, P, and Si at an atom % concentration, e (at %), rangingwithin 0≦e (at %)≦10.

[0020] As a result thereof, the admixture of T allows controllingappropriately a plateau pressure at which the resultant hydrogen storagemetal alloys can absorb, store and release hydrogen.

[0021] The selected compositions for hydrogen storage alloys accordingto the present invention are set forth on the basis of the followingreasoning:

[0022]FIG. 2 depicts a Ti—Cr binary system phase diagram in connectionwith the present invention. As seen in FIG. 2, the BCC phase is presentthroughout all composition ranges in Ti—Cr series at 1643 K (1370° C.)or higher. In light of the atomic radius of Ti (0.147 nm) greater thanthat of Cr (0.130 nm), when the level of Ti increases and the level ofCr lessens, the alloy will increase its BCC phase lattice constant butlower its plateau pressure. Although the plateau pressure of thehydrogen storage alloy varies depending on the alloy-operatingtemperature, the ratio of Ti to Cr may vary in order to acquire adesired operating temperature. Therefore, a suitable Ti/Cr ratio can beoptionally selected. In embodiments as described herein below, thestarting composition is set to the extent of Ti₄₀Cr₆₀ so as to acquire asuitable plateau pressure at 40° C. (313K), but this invention is notlimited to. The plateau pressure of the hydrogen storage alloys variesdepending on their alloy-operating temperature, and the plateau pressurecan be controlled in Ti—Cr—M-based hydrogen storage alloys by changingthe ratio of Ti to Cr. The plateau pressure is remarkably raised whenthe Cr level “a” exceeds 80 at % but on the contrary extremely loweredwhen it is below 20 at %, thereby leading to a poor practicability.Accordingly, the Ti/Cr ratio which is suited for a desired workingtemperature may be selected within a range of 20≦a(at %)≦80.

[0023] Further, since element V has an atomic weight approximatelyequivalent to that of Ti or Cr though it is expensive, an increase inmolecular weight for alloy products can be minimized even if itssubstitution quantity is increased. Therefore, there is an advantagethat an amount of occluded hydrogen per unit weight will not be reducedmuch. In contrast, since Mo and W each have a great BCCstructure-forming property to Ti—Cr binary alloys, the admixture of Moand/or W with the Ti—Cr binary alloy facilitates the formation of BCC inalloy products. Therefore, Mo and W are effective. However, an excessiveamount of admixed Mo and W will lead to the deterioration of hydrogenadsorption and storage characteristics because of heavy elements eachhaving a large atomic weight. Hence, to utilize both the advantages, anovel composition is invented wherein part of expensive V is replacedwith Mo and/or W, i.e., an alloy composition of the followingfundamental formula:

Ti_((100-a-0.4b))Cr_((a-0.6b))V_((b-c))M_(c)

[0024] wherein 20≦a (at %)≦80, 0≦b (at %)≦10, and 0≦c (at %)≦5, and M isat least one element of Mo and W, is provided. This composition has agreat practicability in cost, hydrogen storage capacity and BCCstructure-forming capability. Similarly to the above, the admixture ofsubstituent element T in connection with this composition is alsoeffective in adjusting the plateau pressure wherein T is at least one ormore elements selected from the group consisting of Nb, Ta, Mn, Fe, Al,B, C, Co, Cu, Ga, Ge, Ln (various lanthanoid metals), N, Ni, P and Si.

[0025] Alloys having a composition with a low level of these elements Moand W are hardly formed in the structure of BCC as pointed out in theprior art. As apparent from the phase diagram of a Ti—Cr binary alloy(FIG. 2), this is attributable to the fact that a temperature range foraffording the BCC structure is too narrow throughout the Ti—Cr admixtureratios wherein temperature and pressure ranges at which the hydrogenstorage alloy can work will be within practicable values, i.e., at theCr level of 20 to 80 at %.

[0026] As seen in the aforementioned phase diagram (FIG. 2), however,for example, when the level of Cr is gradually reduced from 60 at % (ithas the same meaning as the level of Ti gradually increases from 40 at%), a temperature range eligible for giving a BCC structure wouldexpand. This is presumably attributed to the following: since the Lavesphase is represented by a composition of an AB₂ type and the atomicradius ratio of A to B (rA:rB)=about 1.225:1 is necessary for forming anideal geometric structure in such a composition while the atomic radiusratio of Ti to Cr (both of which are used according to the presentinvention) is 1.13:1, which is far different from the above ideal valueand unsuitable for forming the ideal Laves phase structure, Ti willquantitatively increase, and invade B sites in apparently morequantities whereby consequently the atomic radius ratio at A sites willbecome closer to that at B sites, thereby inhibiting the formation ofLaves phases.

[0027] Now, by developing such ideas, when an element having an atomicradius smaller than that of the A site but larger than that of the Bsite is admixed therewith for substitution, the formation of Laves phasecan be inhibited even if the substituent element invades the A site andalso even if the B site is replaced.

[0028] Hence, it has been thought that there is a possibility ofenabling a BCC formation in alloy products similarly to the above V caseas well as the Mo or W case and therefore an element X (its atomicradius is smaller than that at the A site (Ti) but larger than that atthe B site (Cr)) can be added to the alloy to expand a temperature rangeeligible for forming BCC whereby a hydrogen storage alloy may beproduced with a BCC structure in a more stable fashion.

[0029] The element X having an atomic radius smaller than that at the Asite (Ti) but larger than that at the B site (Cr) includes, in additionto the above Mo, W, and V, for example, at least one or more elementsselected from the group consisting of Al, Ru, Rh, Pt, Nb, Ta, Sb andothers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a flow chart illustrating a process for producing thehydrogen storage alloy according to an embodiment of the presentinvention.

[0031]FIG. 2 depicts a Ti—Cr binary system phase diagram.

[0032]FIG. 3 is an X-ray diffraction pattern each of as heat-treated (at1400° C. for 1 hour) alloys Ti₃₇ ₅Cr₆₀V₂ ₅ and Ti₃₇ ₅Cr₆₀Mo₁ ₂₅V₁ ₂₅.

[0033]FIG. 4 is a graph showing hydrogen absorption and desorptioncharacteristics (at 40° C.) for as heat-treated alloyTi_(42.5)Cr_(57.5).

[0034]FIG. 5 is an X-ray diffraction pattern of as heattreated (at 1400°C. for 1 hour) alloy Ti₄₀Cr₅₇ ₅Al₂ ₅.

[0035]FIG. 6 is a graph of hydrogen absorption and desorptioncharacteristics (release curve, 40° C., 5th cycle) upon application ofdifferential temperature method to alloy V_(x)Ti₃₇ ₅Cr_(62 5-x).

[0036]FIG. 7 is a graph showing hydrogen absorption and desorptioncharacteristics (at 40° C.) for as heat-treated (at 1400° C. for 1 hour)alloy V_(x)Ti_(37.5)Cr_(62 5-x).

[0037]FIG. 8 is a graph showing the relationship of admixed amounts of Vversus hydrogen absorption and desorption characteristics for aTi—Cr—V(—Mo) alloy.

[0038]FIG. 9 is a graph showing the relationship of admixed amounts ofMo versus hydrogen absorption and desorption characteristics for aTi—Cr—Mo(—V) alloy.

[0039]FIG. 10 is a graph showing hydrogen absorption and desorptioncharacteristics (at 40° C.) for as heat-treated (at 1400° C. for 10 min)alloy Ti₃₈Cr₅₄V₅Mo₂Ta₁.

[0040]FIG. 11 is a graph showing hydrogen absorption and desorptioncharacteristics (at 40° C.) for as heat-treated (at 1400° C. for 10 min)alloys Ti_(37.5)Cr₅₈V_(2.5)W₂ (this invention) and Ti₃₇ ₅Cr_(51.5)V₅W₆(Comparative Example).

[0041]FIG. 12 is a graph showing hydrogen absorption and desorptioncharacteristics (at 40° C.) for as heat-treated (at 1400° C. for 10 min)alloys Ti₃₇ ₅Cr₅₇V₂ ₅Mo₁Al₂ and Ti_(37.5)Cr₅₅V₅Mn_(2.5).

PREFERRED EMBODIMENTS FOR EXECUTING THE INVENTION

[0042] Described below are the hydrogen storage metal alloys of thepresent invention and processes for the production of the said metalalloys in detail, relying on experiments conducted by the presentinventors.

[0043] First, FIG. 1 is a flow chart showing a preferred embodiment ofthe process for producing the hydrogen storage alloys according to thepresent invention. Such a process is applied to the production ofhydrogen storage alloys used in the experiments conducted by the presentinventors as described herein below.

[0044] In this process for the production of hydrogen storage metalalloys, each constituent metal for an intended hydrogen storage alloy(for example, each of Ti, Cr and V where Ti₃₇ ₅Cr₆₀V_(2.5) is preparedas a product) is weighed at an amount corresponding to each compositionratio so as to bring the total weight of a resultant ingot to 12.5 g.

[0045] Each individual metal thus weighed is placed in an arc meltingplant (not depicted), subjected to repeated treatments (melting-stirring←→ solidification) predetermined times (which may vary depending on thenumber of constituent elements in experiments but be usuallyapproximately 4 to 5 times) in an argon atmosphere of about 40 kPa withscrupulous care to elevate a uniformity and the resultant homogenizedingot is then maintained at a temperature region just lower than themelting point of its melt for a predetermined time to accomplish theheat treatment.

[0046] Since a temperature region at which BCC forms are produced ispresent at an area just below the melting temperature owned by an alloyhaving a target composition as shown in the above FIG. 2 (phasediagram), the heat-treatment may be preferably effected at such atemperature region at which the BCC is produced and just below themelting temperature. For example, in the aforementioned compositioncontaining about 60 at % of element Cr, the heat-treatment is preferablyeffected by retaining the molten alloy at about 1400° C. It is alsopreferable to select a suitable heat treating temperature fromtemperature areas at which a target alloy is produced in the form of BCCand just below the melting temperature of the target alloy, depending onits alloy composition. Among temperature areas at which the BCC isproduced and just below the melting temperature thereof, it should benoted that it will take a longer time to accomplish the heat-treatmentwhen the treatment temperature is too low (about 1000° C. or lower)while it will take only a short time but the heating cost will beincreased much when it is too high. Therefore, by taking the foregoingpoints into account, it is preferable to select a heat-treatingtemperature.

[0047] When a heat-treating time is too short, it will be impossible toaccomplish the formation of sufficient BCC structure phases, and when itis too long, not only the heat-treating cost will be increased but alsoan adverse action will appear whereby heteromorphic phases would beprecipitated to deteriorate the hydrogen absorption and desorptioncharacteristics. Accordingly, the operation period can be suitablyselected on the basis of a selected heat-treating temperature, but itmay be preferably within a range of from 1 min to 1 hour.

[0048] In the embodiments, after melting ingots, alloys per se aresubjected to the aforementioned heat treatment without making anyshapes. Since such a process does not require that cooled alloys arere-heated but allows producing efficiently alloy products having a BCCstructure phase, it is preferable but the present invention is notlimited to. For example, it may be preferred that molten alloys areshaped once by methods such as strip casting, single rolling andatomizing to afford plates, ribbons or powders, then cooled and theresultant alloys each having either the BCC phase+the Laves phase or theLaves phase alone are subjected to the aforementioned heat treatment soas to give products each having the BCC structure phase as the mainphase.

[0049] Among these alloys, alloys (ingots) heat-treated to an extentthat the BCC structure phase takes place as the main phase are rapidlycooled by dipping into ice water to give alloy products wherein theabove BCC structure phase is still retained. In the embodiments, theaforementioned rapid cooling (quenching) is carried out by dipping intoice water, but the present invention is not limited to. Any can beoptionally selected for these cooling methods. However, since the volumeratio of BCC structure phase varies depending on cooling rates and aslow cooling rate leads to a decrease in the BCC structure phase volumeratio, it is desired that the alloy is quenched preferably at a coolingrate of 100 K/sec or more.

[0050] Although the alloys of the present invention have a compositionapt to induce a spinodal decomposition readily, it is defined that,because spinodal decomposing tissues cause deterioration of alloy'shydrogen absorption and desorption characteristics, they are permittedto the extent there is an unavoidable formation.

[0051] The aforementioned V has an atomic weight approximatelyequivalent to that of Ti or Cr. Although V is expensive, a change(increase) in molecular weight for alloy products is minimized even whenan amount of substituents increases. Therefore, there are advantagesthat amounts of occluded hydrogen do not reduce very much. Accordingly,in order to produce BCC mono phase alloys with a high capacity bymelting a large amount of alloys followed by rapidly cooling (quenching)and, if necessary, heat-treatments, it is forecasted that V may beeffectively admixed therein in combination with at least one memberselected from the aforementioned Mo, W, etc. Thus, for theaforementioned low V level Ti—Cr—V alloys, which have beenconventionally considered to be hardly produced in a BCC phase form,their efficacies are examined and proved in case where a replacementwith Mo partially takes place.

[0052] An X-ray diffraction pattern each of as heat-treatedTi_(37.5)Cr₆₀V₂ ₅ and Ti_(37.5)Cr₆₀Mo₁ ₂₅V_(1.25) alloys is shown inFIG. 3. Reflections by the Laves phase are observed for the heat-treatedalloy Ti_(37.5)Cr₆₀V₂ ₅ as shown in FIG. 3 and the hydrogen adsorptionand desorption characteristics remain to an extent of 2.6%. However, ithas been found that the heat-treated alloyTi_(37.5)Cr₆₀Mo_(1.25)V_(1.25) wherein V is partially replaced with Moare almost in the form of a BCC mono phase and its hydrogen adsorptionand desorption characteristics are improved to be an extent of about 2.7wt %. In this way, V can be admixed therein in combination with Mo (alsoW) so as to reduce an amount of expensive V to be admixed together witha reduction in amounts of Mo (and/or W) to be admixed, with the resultthat the occupied volume ratio of BCC phases will increase together withthese admixtures, thereby leading to an increase in hydrogen adsorptioncapacity. Therefore, it can be said that the admixture of V incombination with Mo (and/or W) is a preferable technique for producinginexpensive hydrogen storage metal alloys with a high capability ofabsorbing and storing hydrogen.

[0053] In the Ti—Cr-based alloys, it is further supposed that theformation of the BCC phase is facilitated more as its structure is moredistant from the ideal geometric structure of the Laves phase (TiCr₂)represented by the AB₂ type composition. Accordingly, the BCC phase canbe easily formed by the admixture of a readily solid-soluble elementeffective to avoid the ideal atomic radius ratio 1.225:1 between boththe constituent atoms, A and B, for the Laves phase. When thesubstitution is performed with an element having an atomic radiussmaller than the site A but larger than the site B, the substituentelement can inhibit the Laves phase formation even if it intrudes intothe site A and similarly inhibit the Laves phase formation even if itsubstitutes the B-site, so that the formation of the BCC type phase willbe facilitated. Such elements include, for example, Al, Ru, Rh, Pt, Nb,Ta, Sb and the like, in addition to the above Mo, W and V.

[0054] Thus, there has been no report that, in view of such atomicradiuses, the Ti—Cr binary alloy was subjected to the formation of a BCCmono phase or the facilitation of a BCC phase formation. This is one ofthe grounds for supporting the novelty of the present invention. Thehydrogen absorption and desorption characteristics of as heat-treatedalloy Ti₄₂ ₅Cr_(57.5) are shown in FIG. 4. Its hydrogen storage capacityis 2.6 wt % or more. Distinctively from conventional Ti—Cr Laves alloysand the like as reported in the prior art, these results evidence thatthe BCC phase occurring in the Ti—Cr binary alloy has advantageoushydrogen adsorption and desorption characteristics.

[0055] While the BCC type phase appearing in ternary system alloys suchas Ti—Cr—V and Ti—Cr—M (M=Mo or W) alloys is intended in JP, A,10-121180, JP, A, 10-158755 and JP, A, 11-106859, the following has beenexperimentally proved according to the present invention:

[0056] Ti—Cr—V alloys and Ti—Cr—Mo (W) or Ti—Cr-(V or Mo) alloysaccording to the present invention are produced in the form of a BCCmono phase or in a BCC main phase form at a range substantially close tothe Ti—Cr binary alloy wherein an extremely micro amount of V, Mo, W,etc. is admixed, thereby exerting excellent hydrogen adsorption anddesorption characteristics. This is attributed to the fact that the BCCphase of such Ti—Cr binary alloys exerts its excellent hydrogenadsorption and desorption characteristics.

[0057] An X-ray diffraction pattern of as heat-treated alloys Ti₄₀Cr₆₀and Ti₄₀Cr_(37.5)Al_(2.5) is shown in FIG. 5. It is apparent that theBCC mono phase is almost formed by replacing part of Cr with Al.

[0058] This alloy is realized, by further developing the concept that apreferable Ti—Cr-based alloy is Ti₄₂ ₅Cr₅₇ ₅ alloy rather than Ti₄₀Cr₆₀alloy, i.e., Cr is replaced with Ti having a larger atomic radius thanCr to bring the atomic radius ratio of A to B (rA:rB) to such an extentthat the Laves phase formation will be easily suppressed as shown inTi—Cr series, and using Al (0.143 nm) which has an atomic radius largerthan Cr (0.130 nm) but smaller than Ti (0.147 nm) and can not onlyinhibit the formation of a Laves phase but also reversely promote theformation of BCC even irrespective of which of A and B sites isreplaced. The additive elements having an action similar to Al includeRu, Rh, Pt, Nb, Ta, Sb and the like, as aforementioned, from the pointof atomic radius.

[0059] It has been examined and ascertained herein below that the BCCstructure phase is produced by the aforementioned production processesand experimental results are also shown which support grounds forselecting the above compositions.

[0060] The efficacy of addition of V in combination with Mo to Ti—Cralloys is examined and verified. The quantitatively additive V-dependenthydrogen storage capacity for Ti₄₁ ₇Cr_(58 3-x)V_(v) and Ti₄₁₇Cr_(57 3-x)Mo₁V_(x) alloys when measured at 40° C. is shown in FIG. 8.Although the hydrogen storage capacity is reduced and becomes equivalentto that of the V-free composition when the amount of admixed V exceeds10% in any case, the amount of admixed Mo necessary for providing alarge hydrogen storage capacity can be made small in the compositelyMo-added alloy, as compared with the Mo-free alloy. For total amounts ofadditive Mo and V, a larger hydrogen storage capacity can beadvantageously obtained in the compositely added alloy with a smallamount of the additives.

[0061] The quantitatively additive Mo-dependent hydrogen storagecapacity for Ti₄₁ ₇Cr_(58 3-x)Mo_(x) and Ti₄₁ ₇Cr_(56 3-x)Mo_(x)V₂.alloys when measured at 40° C. is shown in FIG. 9. The amount of admixedMo necessary for providing a large hydrogen storage capacity is reducedwith addition of V at 2 at %. Mo is highly BCC phase producible andextremely effective in obtaining the BCC type phase. However, when theamount of admixed Mo is increased, Mo is apt to segregate in a meltingprocess because the melting point of Mo is extremely high, i.e., 2610°C., as compared with Ti (melting point=1668° C.) and Cr (meltingpoint=1875° C.). Thus, the amount of additive Mo can be furtherminimized by adding a small amount of V (melting point=1890° C.) so asto suppress the segregation.

[0062] The results where Ta is also compositely added to the Ti—Cralloys in combination with both V and Mo are shown in FIG. 10. Since Tais an element having an atomic radius smaller than Ti and larger thanCr, and solid-soluble to any of Cr and Ti, the action of suppressing theformation of the Laves phase via its solid-solution formation in theTi—Cr-based alloy can be expected. This Ti₃₈Cr₅₄V₅Mo₂Ta₁ alloy isprepared by retaining at 1400° C. for 10 min and immediately quenchingin ice water. It has been confirmed from the resultant X-ray diffractionpatterns that this Ti₃₈Cr₅₄V₅Mo₂Ta₁ alloy is composed of the BCC singlephase. Thus, it is also effective for providing a large hydrogen storagecapacity that an element having an atomic radius smaller than Ti andlarger than Cr is suitably added to Ti—Cr—V—Mo alloys to suppress theformation of Laves phases.

[0063] The PCT curves (measured at 40° C.) for alloys comprising V and Wcompositely added to Ti—Cr-based alloys are shown in FIG. 11. It hasbeen confirmed from the resultant X-ray diffraction patterns that eachalloy is composed of BCC mono phases. In the alloys with a large amountof additive W (6%, Comparative Example), the hydrogen storage capacityis remarkably deteriorated, as compared with the alloys with an amountof 2% (this invention). Although W is also an alloy having high BCCforming capability similarly to Mo, the amount of admixed W is limitedto less than 5% since the hydrogen storage capacity is deteriorated whenthe amount of admixed W is too large.

[0064] Al is an element capable of elevating the plateau pressure viaits solid-solution formation in Ti—Cr-based alloys. The resultant PCTcurve (measured at 40° C.) for alloys obtained by adding Al toTi—Cr—V—Mo alloys is shown in FIG. 12. The results for Ti—Cr—V—Mn alloysadmixed with Mn instead of Mo are also shown in FIG. 12. Thus, it isalso effective in the application of materials that the plateau pressureis changed by adding Al, Mn or the like.

[0065] It is reported in Japanese Patent Application No. 11-86866 ((or86866/1999) that hydrogen can efficiently be utilized via applicationsof a difference in temperature, characterized by storing hydrogen at alow temperature in body-centered cubic structure hydrogen storage alloyseach having a two-stage plateau or inclined plateau and elevating thealloy working temperature to a high temperature for at least a period ofhydrogen release process. In case where the differential temperaturemethod is applied to the aforementioned V_(x)Ti_(37.5)Cr_(62 5-x) alloy,its hydrogen absorption and desorption characteristics are shown in FIG.6. It is apparent that the application of the differential temperaturemethod to the alloys of the present invention will lead to a hydrogenstorage capacity of about 3.0 wt %. As compared to FIG. 7, it isobserved that the differential temperature method derives an increase inhydrogen storage capacity at about 0.2 wt %, and it is thereforeexperimentally proved that the differential temperature method iseffective for alloys attained by the present invention. Itspracticability can also be understood.

1. A hydrogen storage alloy having the following characteristics: (1) ithas as its main phase a body-centered cubic structure phase capable ofabsorbing, storing and releasing hydrogen, and (2) it has a compositionof the following general composition formula:Ti_((100-a-0 4b))Cr_((a-0 6b))V_((b-c))M_(c) wherein 20≦a (at %)≦80, 0≦b(at %)≦10, and 0≦c (at %)<5, and m is at least one element selected frommolybdenum (Mo) and tungsten (W):
 2. The hydrogen storage alloyaccording to claim 1 which contains an element X having an atomic radiuslarger than Cr and smaller than Ti at a range of 0≦d (at %)≦20, providedthat d is an atom % concentration (at %) of X.
 3. The hydrogen storagealloy according to claim 1 or 2 which contains at least one or moreelements (T) selected from Nb, Ta, Mn, Fe, Al, B, C, Co, Cu, Ga, Ge, Ln(various lanthanoid metals), N, Ni, P and Si at a range of 0≦e (at%)≦10, provided that e is an atom % concentration (at %) of T.